An active pixel sensor, the fundamental imaging component within a broad variety of video and still-image cameras, is formed by an array of active pixels, each containing a photodiode and a number of pixel-control transistors. Because the photodiode voltage discharges incrementally in response to incident light, a digital image may be captured by exposing a precharged pixel array (i.e., array of active pixels having charged photodiodes) to a light source for a period of time and then reading out and digitizing the residual photodiode voltages.
Because the photodiode discharges incrementally in response to photon strikes, the dynamic range of a conventional active pixel (i.e., the difference between the brightest and dimmest light levels that can be represented by the digitized output) is a function of the photodiode size (i.e., a larger photodiode can hold a greater initial charge), and the resolution of the analog-to-digital converter (ADC) used to digitize the photodiode output. Accordingly, the relatively small photodiodes and low-resolution ADCs typically deployed in low-end image sensors (e.g., in cell-phone cameras) tend to have commensurately low dynamic range, saturating quickly in bright light. Unfortunately, even in high-end digital sensors having relatively large photodiodes and complex high-resolution ADCs, the linear relationship between luminance and photodiode voltage yields a substantially lower dynamic range than, for example, the human eye.
One approach to extending the dynamic range of an active pixel involves temporally oversampling the pixel during an exposure interval to determine if the photodiode is approaching saturation and, if so, resetting the photodiode and counting the reset event. At the conclusion of the exposure interval, the number of reset events and the final sample of the photodiode voltage may be used to construct a pixel value for the entire exposure interval. Viewing the photodiode as a well or bucket for collecting photons, each oversampling/reset event constitutes an opportunity to re-use the well (and is thus referred to as “recycling the well”) so that the effective size of the well and thus the dynamic range of the image sensor corresponds, approximately, to the product of the physical size of the photodiode and the temporal oversampling factor.
One problem with sample-driven well-recycling is that the photodiode may saturate between sampling/reset instants, resulting in an unknown number of uncounted photon strikes and thus distortion in the final output value. In one well-recycling variant, circuitry is provided within each active pixel to monitor the photodiode voltage and asynchronously “self-reset” the photodiode upon reaching a near-saturation threshold, thereby avoiding saturation. Because the photodiode discharge is monotonic, occurrence of an asynchronous reset may be inferred (i.e., detected and counted) by an increase in the photodiode voltage read-out and digitized in successive sampling instants. Unfortunately, the relatively slow and power-hungry ADC operation limits the number of samples that may be acquired in a given exposure interval. Consequently, the improvement in dynamic range tends to be modest and comes with a significant power consumption penalty.